Most TCP implementations use a sliding window mechanism for sending data. Multiple data segments are sent in succession without waiting for an acknowledgment for each segment.
The sliding window algorithm uses a lot of 32-bit operations and because 32-bit arithmetic is fairly expensive on most 8-bit CPUs, uIP does not implement it. Also, uIP does not buffer sent packets and a sliding window implementation that does not buffer sent packets will have to be supported by a complex application layer. Instead, uIP allows only a single TCP segment per connection to be unacknowledged at any given time. lwIP, on the other hand, implements TCP's sliding window mechanism using output buffer queues and therefore does not add additional complexity to the application layer.
It is important to note that even though most TCP implementations use the sliding window algorithm, it is not required by the TCP specifications. Removing the sliding window mechanism does not affect interoperability in any way.